This invention relates to a color image sensor of the type that reads color images with the aid of filters that absorb light of different colors (e.g. red, green and blue) and that are provided over arrays of light-receiving devices formed in a plurality of rows on a common substrate. More particularly, this invention relates to a color image sensor of high reliability that is improved in the switching characteristics of thin-film transistors.
A typical example of conventional color image sensors that have arrays of light-receiving devices formed in a plurality of rows on a common substrate is shown in FIG. 6 which is a schematic plan view and in FIG. 7 which is a cross-sectional explanatory view. As shown in FIG. 6, the sensor portion comprises a substrate 1 on which arrays of light-receiving devices 11 which are assemblies of light-receiving devices 11' are formed in the main scanning direction, with the arrays 11 consisting of three arrays, 11a, 11b and 11c, which are juxtaposed parallel to one another in the subsidiary scanning direction. In the case shown, arrays 11a, 11b and 11c are adapted to read red, green and blue light, respectively. The respective light-receiving devices are connected to thin-film transistors 12' that are switching elements for transferring electric charges, so that arrays of charge transfer portions 12a, 12b and 12c are formed in correspondence to respective arrays 11a, 11b and 11c of light-receiving devices. Signal lines drawn from the charge transfer portions 12 are connected to a multilevel wiring 13.
As shown in FIG. 7, each of the light-receiving devices 11' comprises a metallic electrode 21, a photoconductive layer 22 and a transparent electrode 23 that are formed in superposition on the substrate 1 to compose a sandwich structure. The metallic electrode 21 serves as a lower common electrode and is made of a chromium (Cr) layer in strip form; the photoconductive layer 22 is made of hydrogenated amorphous silicon (a-Si:H) segmented for each light-receiving device; and the transparent electrode 23 is an upper electrode that is made of similarly segmented indium tin oxide (ITO).
As also shown in FIG. 7, each of the thin-film transistors 12' comprises a gate electrode 24, a gate insulating layer 25, a semiconductor active layer 26, a channel protective layer 27, an ohmic contact layer 30, a drain electrode 28, a source electrode 29, an inter-level insulating layer 31, a light-shielding metallic layer 32', and a wiring layer 32, and these components are assembled on the substrate 1 to form a reverse-staggered transistor. The gate electrode 24 is formed of chromium (Cr1); the gate insulating layer 25 is a silicon nitride film (SiNx) that covers the gate electrode 24; the semiconductor active layer 26 is made of hydrogenated amorphous silicon (a-Si:H) deposited over the gate insulating layer 25; the channel protective layer 27 is formed of SiNx in such a way as to be in registry with the gate electrode 24; the ohmic contact layer 30 is made of n.sup.+ hydrogenated amorphous silicon (n.sup.+ a-Si:H) and provided on the semiconductor active layer 26; the drain electrode 28 and the source electrode 29 are formed of chromium (Cr2) to cover the ohmic contact layer 30; the inter-level insulating layer 31 is formed of polyimide to cover the channel protective layer 27; the light-shielding metallic layer 32' is made of aluminum (Al) to shield the channel protective layer 27 from light; and the wiring layer 32 is connected to the drain electrode 28 and the source electrode 29.
FIG. 8 is a schematic plan view of the filter portion of the image sensor under consideration. As shown, it comprises another transparent thin insulating substrate 2 that has color filters 34 formed thereon for performing color separation on image information. The color filters are adapted to absorb light of predetermined colors, say, red, green and blue. These filters 34 correspond to each array of light-receiving devices 11 in terms of both length and width and are formed in strip in the main scanning direction. They are arranged in three rows in the subsidiary scanning direction, i.e., color filters 34a, 34b and 34c which are associated with red, green and blue colors.
The substrates 2 and 1 are bonded to each other in such a way that the color filters 34 for different colors which are formed on the substrate 2 will come in registry with the top surfaces of the respective arrays of light-receiving devices 11 which are formed on the substrate 1 (e.g. the red color filter 34a on top of the array 11a, the green color filter 34b on top of the array 11b, and the blue color filter 34c on top of the array 11c), whereby a desired color image sensor is completed.
The color image sensor of the construction described above is operated in the following manner: when the light reflected from the document surface passes through the color filters 34, only the light of specific wavelength components that are determined by the colors of the respective filters are picked up to reach the working portions of the light-receiving devices 11', where electric charges are generated in amounts that depend on the illuminance of received light. Stated more specifically, the array 11a of light-receiving devices will respond to red light, the array 11b to green light and the array 11c to blue light, thereby producing electric charges and, as the thin-film transistors 12' are turned on or off, image information for red, green and blue colors are read sequentially over common signal lines. Image signals for individual colors are stored temporarily in a memory external to the sensor for synthesis of image data.
If the semiconductor active layer 26 in each thin-film transistor 12' is an a-Si:H layer as in the case of the conventional color image sensor described above, the photoelectric effect of the a-Si:H layer causes electric charges to be generated when the channel portion of each thin-film transistor 12' is illuminated with light and the leakage current which flows in an OFF state will increase to such an extent that the switching characteristics of the thin-film transistors will deteriorate. To avoid this problem, the light-shielding metallic layer 32' which is the same Al metallic layer as the wiring layer 32 has been formed on top of the channel of each thin-film transistor 12' as shown in FIG. 7.
However, the conventional color image sensor is already complex in structure since arrays of light-receiving devices are arranged in three rows and the respective light-receiving devices are in one-to-one correspondence with thin-film transistors as they are connected to each other. Thus, the system layout becomes more complicated if one attempts to form light-shielding metallic layers that cover the channel portions of the individual thin-film transistors. As a further problem, it is difficult to achieve interconnection in such a way as to keep the light-shielding metallic layers at a constant potential.
The use of a metal as the material of the light-shielding layers unavoidably results in a structure in which the metallic light-shielding layer is superposed on the thin-film transistor as they are separated only by an inter-level insulating layer and, hence, stray capacitance will develop between the metallic light-shielding layer and each of the drain and source electrodes of the thin-film transistor. If unwanted stray capacitance is loaded on the source electrode, the output of electric charges to common signal lines will decrease and the resulting lower sensitivity will lead to impaired performance of the image sensor.